1. Field of the Invention
The present invention relates to an optical cable for telecommunications and/or for data transmission, which is particularly suitable for installations of the terrestrial type.
In particular, the present invention relates to an optical cable comprising at least one element for the transmission of optical signals and a multilayer protective structure arranged in a position radially external to said at least one element, said protective structure endowing said cable with high mechanical strength, mainly with respect to lateral stresses such as, for example, impact and/or compression.
In the present description, and in the claims that follow, the term “element for the transmission of optical signals” means any transmission element comprising at least one optical fiber. Therefore this term identifies both a single optical fiber, and a plurality of optical fibers, possibly combined together to form a bundle of optical fibers or arranged parallel to one another and covered with the same covering to form a tape (ribbon) of optical fibers.
Moreover, this term is also intended to include configurations that are more complex than a single optical fiber or a plurality of optical fibers. Therefore said term includes one or more optical fibers arranged inside a containment structure, for example a tubular element, a sheath, a microsheath or a grooved core. Preferably, said tubular element, sheath or microsheath is made of a polymeric material. Moreover, said tubular element, sheath or microsheath can be contained within the grooves possessed by said grooved core.
In accordance with current terminology, one or more of said containment structures, enclosing one or more optical fibers therein, is conventionally indicated by using the term “optical core”. For example, an optical core can be formed by a plurality of tubular elements made of polymeric material, stranded together around a central reinforcing element. Optionally, said optical core can in addition include fabrics and/or tapes, for example waterswallable fabrics and/or tapes, which can be used for making the aforesaid stranding.
In the present description, and in the claims that follow, the term “optical cable” means a fiber-optic cable, i.e. a cable provided with at least one element for the transmission of optical signals.
2. Description of the Related Art
With particular reference to applications of the terrestrial type, during the step of laying and/or the step of transport of an optical cable, the latter may be subjected to accidental impacts and/or compressions, due for example to the falling of large lumps of excavated material or of tools used during cable installation, onto the cable arranged in the cable-laying trench.
This accidental falling can cause not only a violent and substantially instantaneous impact action on the cable, but also a constant compressive action for a longer time, until the caved-in excavated material and/or the excavating equipment that is inadvertently resting on or has fallen on the surface of the cable is removed, therefore exerting a continuous compressive action.
Accordingly, to preserve its structural integrity, an optical cable is generally provided with at least one protective covering that is able to endow said cable with suitable mechanical strength.
In fact it must be emphasized that, in the absence of suitable measures for protection, any mechanical actions of impact and/or of compression (crushing), even of moderate magnitude, acting on the core of an optical cable, can be transmitted directly to the individual optical fibers contained inside said core, first of all causing an increase in attenuation of the optical signal transmitted by said fibers and then, if the phenomenon persists, breakage of said fibers.
In fact, the optical fibers, by themselves or even arranged inside a containment structure as above, have limited mechanical strength and are particularly sensitive to mechanical actions acting on them from outside, even though of low intensity.
Minimal deformations of said fibers as well as of the containment structures of the latter, especially in the case when the deformed structures come into contact with the fibers, lead to deformation of the fibers themselves with consequent degradation of their transmission characteristics. The phenomenon of micro deformation of the structure of a fiber that leads to attenuation of the signal being transmitted by the fiber is commonly indicated by the term “microbending”.
For the purpose of endowing an optical cable with a predetermined mechanical strength suitable to resist external forces, such as impact and/or compression, it is known in the art the use of armours, generally metallic, that are arranged in a position radially external to the optical core for protecting the latter.
In general, said armours are provided by applying a metallic strip, preferably corrugated, formed longitudinally around the cable or by stranding a plurality of metal wires wound in a helical configuration. In this connection, see for example document U.S. Pat. No. 4,491,386.
The provision of an optical cable with a cushioning layer made of a sufficiently soft material, for example a foamed plastic material, for protecting the optical fibers of said cable, is also known in the prior art.
Said cushioning layer can be combined with various constitutive elements of the cable. For example, it can be arranged inside the optical core to surround each individual tubular element containing the optical fibers or to form the tubular element itself, or it can be arranged externally to said optical core, for example in a position directly beneath the external polymeric sheath of the cable.
For example, document GB-1,451,232 envisages the use of a layer of compressible material that is able to reduce the deformation of the fibers if the optical cable is subjected to stressing such as to submit one or more fibers to longitudinal tensile stresses. In the presence of such a stressed state, one or more optical fibers move transversely to the cable. However, since said fibers come into contact with said compressible cushioning layer and compress it inwards, partially penetrating into it, the deformation of the fibers is reduced considerably.
A further example of a cushioning layer made of soft material, for example of foamed plastic material with an elastic modulus preferably below 100 MPa, is described in document DE-3,107,024. This layer is used for protecting the optical fibers against transverse forces acting laterally on the optical cable.
Further types of embodiments involving the use of cushioning layers of the type as above are described, for example, in documents GB-2,159,291 and GB-2,184,863.
A further solution aiming to increase the mechanical properties of an optical cable, in particular impact strength and compressive strength, is described in document U.S. Pat. No. 4,770,489.
Said document relates to an optical cable having improved tensile strength and a wider range of working temperatures owing to the presence of at least one rigid tension member, made of fiber-reinforced plastic material, possessing a high Young's modulus and low coefficient of thermal expansion, it being possible to make said element either in the form of a covering, or in the form of one or more threads that extend longitudinally to the cable and are incorporated within the latter.
Said document points out that, for the purpose of endowing the optical cable with high impact strength and compressive strength, as well as to allow a high installation tension without lowering the transmission capabilities of the cable itself, the latter should be provided with at least one rigid tension member as above and with an outer sheath of reinforced plastic material, for example reinforced with fibers such as fibers of glass, Kevlar®, fibers of graphite embedded in an epoxy resin matrix.
In accordance with some embodiments that are described, document U.S. Pat. No. 4,770,489 envisages, furthermore, that the optical cable can be provided with a cushioning layer that is able to increase the characteristics of bending and curvature of said cable. Preferably said cushioning layer is made of a fibrous material such as Kevlar® or a polypropylene yarn. If required, said layer can be made of a expanded material, for example polyethylene.
Document WO 00/05730, in the name of the same Applicant, relates to a hybrid optical/electric cable suitable for installation along overhead lines for telecommunications and for electric power distribution, especially at medium voltage.
Said cable comprises a three-phase electric cable with insulated conductors which is wound around a carrying rope including an optical core enclosed in a tubular structure that is resistant to transverse compression.
Generally, said tubular structure consists of a metal sheath (which may also be resistant to corrosion, for example aluminium or stainless steel) or of high-modulus polymeric material (for example polypropylene, modified polypropylene, polybutylene terephthalate (PBT), polyether-imides, polyether-sulphones).
Said tubular structure can, moreover, consist of a foamed polymeric material of the type described in patent application WO 98/52197, in the name of the same Applicant, capable of dissipating the energy that derives from transverse compressive forces that can arise during production, installation and/or operation of the cable and, as mentioned above, that can damage the optical fibers with consequent attenuation of the optical signal being transmitted.
Said compressive forces can be caused either by the insulated phase conductors, which, under the action of a high tensile force, compress the carrying rope and hence the optical fibers contained therein, or by the cable supporting structure which, when placed under tension, tends to reduce its own diameter and hence the internal space that accommodates said fibers.
According to a particular embodiment described in the aforesaid document WO 00/05730, said hybrid cable includes an optical core provided with a reinforcing member around which a plurality of tubular elements are arranged for containing the optical fibers embedded in a buffering filler.
In accordance with said embodiment, the tubular elements are held in position around said reinforcing member, if necessary stranded together according to a preferred helical configuration, by means of one or more tapes which, in addition to performing the binding of said core, can also perform mechanical and/or thermal functions.
In light of the solutions known in the prior art, the Applicant has perceived the need to provide an optical cable, particularly suitable for applications of the terrestrial type, endowed with high mechanical properties in terms of impact strength and compressive strength and which, at the same time, is able to guarantee high performance both in terms of lightness and of flexibility.
In fact the Applicant has found that the solutions of the prior art present several disadvantages.
For example, a cable that is provided with a metallic armour has a considerable increase in weight, which affects not only the step of installation of said cable, but also the step of transport of the cable with an inevitable increase in costs.
Moreover, a cable provided with the aforesaid armour has high flexural rigidity which, combined with said increase in weight, significantly limits the length of the cable to be installed, especially in the case of installations in the ground, inside of existing conduits.
Finally, there are quite considerable drawbacks to providing a metallic armour in the structure of a cable. For example, in the case of buried cables, the presence of metallic elements requires earthing of the cable for reasons of safety and its protection by means of a guard rope suitable to preserve the cable by acting as a sacrificial element. Therefore, this means that both the method of production and the method of installation of the cable are complex and, as a consequence, burdensome both in economic terms and in terms of time.
The solution described in document U.S. Pat. No. 4,770,489 is particularly complex as it envisages the positioning of several rigid tension members to be arranged inside the structure of a cable.
Furthermore, that solution causes a significant increase in weight as well as in flexural rigidity of said cable.
With regard to the solutions known in the art that envisage the use of a cushioning layer for protecting the optical fibers possessed by an optical cable, the Applicant has found that, in view of the softness of the material used, said layer is not able to guarantee sufficient protection when impacts and/or compressions of a high magnitude, such as those caused for example by the falling of excavation debris placed at the edge of the laying trench, accidentally impinge on a cable arranged at the bottom of said trench.
The Applicant, starting from the particular embodiment described in document WO 98/52197, mentioned above, has found that it is possible to optimize the impact strength of an element for the transmission of optical signals arranged in a position radially internal to a covering layer of foamed polymeric material.